<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style>
        body>div{
            width: 300px;
            background-color: rgba(0,0,0,0.3);
            padding: 10px;
            position: relative;
        }
        .input{
            bottom: 0;
            padding: 10px 20px;
            width: 260px;

        }
        .imgs{
            /*设置为绝对定位,以div为参照物*/
            position: absolute;
            right: 40px;
            top: 14px;
        }
        p{
            font-size: 12px;
            color: red;
            margin: 0;
        }
    </style>

</head>
<body>
<div>
    <input type="text" placeholder="请输入用户名" class="input">
    <img src="img.png" class="imgs">
    <p>用户名不能为空</p>
</div>
</body>
</html>